[Misc] add Haystack integration (#18601)

Signed-off-by: reidliu41 <reid201711@gmail.com>
Co-authored-by: reidliu41 <reid201711@gmail.com>

docs/deployment/frameworks/haystack.md

@@ -0,0 +1,60 @@
+---
+title: Haystack
+---
+[](){ #deployment-haystack }
+
+# Haystack
+
+[Haystack](https://github.com/deepset-ai/haystack) is an end-to-end LLM framework that allows you to build applications powered by LLMs, Transformer models, vector search and more. Whether you want to perform retrieval-augmented generation (RAG), document search, question answering or answer generation, Haystack can orchestrate state-of-the-art embedding models and LLMs into pipelines to build end-to-end NLP applications and solve your use case.
+
+It allows you to deploy a large language model (LLM) server with vLLM as the backend, which exposes OpenAI-compatible endpoints.
+
+## Prerequisites
+
+- Setup vLLM and Haystack environment
+
+```console
+pip install vllm haystack-ai
+```
+
+## Deploy
+
+- Start the vLLM server with the supported chat completion model, e.g.
+
+```console
+vllm serve mistralai/Mistral-7B-Instruct-v0.1
+```
+
+- Use the `OpenAIGenerator` and `OpenAIChatGenerator` components in Haystack to query the vLLM server.
+
+```python
+from haystack.components.generators.chat import OpenAIChatGenerator
+from haystack.dataclasses import ChatMessage
+from haystack.utils import Secret
+
+generator = OpenAIChatGenerator(
+    # for compatibility with the OpenAI API, a placeholder api_key is needed
+    api_key=Secret.from_token("VLLM-PLACEHOLDER-API-KEY"),
+    model="mistralai/Mistral-7B-Instruct-v0.1",
+    api_base_url="http://{your-vLLM-host-ip}:{your-vLLM-host-port}/v1",
+    generation_kwargs = {"max_tokens": 512}
+)
+
+response = generator.run(
+  messages=[ChatMessage.from_user("Hi. Can you help me plan my next trip to Italy?")]
+)
+
+print("-"*30)
+print(response)
+print("-"*30)
+```
+
+Output e.g.:
+
+```console
+------------------------------
+{'replies': [ChatMessage(_role=<ChatRole.ASSISTANT: 'assistant'>, _content=[TextContent(text=' Of course! Where in Italy would you like to go and what type of trip are you looking to plan?')], _name=None, _meta={'model': 'mistralai/Mistral-7B-Instruct-v0.1', 'index': 0, 'finish_reason': 'stop', 'usage': {'completion_tokens': 23, 'prompt_tokens': 21, 'total_tokens': 44, 'completion_tokens_details': None, 'prompt_tokens_details': None}})]}
+------------------------------
+```
+
+For details, see the tutorial [Using vLLM in Haystack](https://github.com/deepset-ai/haystack-integrations/blob/main/integrations/vllm.md).